Unterminated strings are still matched as strings. JavaScript strings cannot
contain (unescaped) newlines, so unterminated strings simply end at the end of
the line. Unterminated template strings can contain unescaped newlines, though,
so they go on to the end of input.

Unterminated multi-line comments are also still matched as comments. They
simply go on to the end of the input.

Unterminated regex literals are likely matched as division and whatever is
inside the regex.

Invalid ASCII characters have their own capturing group.

Invalid non-ASCII characters are treated as names, to simplify the matching of
names (except unicode spaces which are treated as whitespace).

Regex literals may contain invalid regex syntax. They are still matched as
regex literals. They may also contain repeated regex flags, to keep the regex
simple.

A human can easily understand that in the number line we’re dealing with
division, and in the regex line we’re dealing with a regex literal. How come?
Because humans can look at the whole code to put the / characters in context.
A JavaScript regex cannot. It only sees forwards.

When the jsTokens regex scans throught the above, it will see the following
at the end of both the number and regex rows:

/ 2/g

It is then impossible to know if that is a regex literal, or part of an
expression dealing with division.

Here is a similar case:

foo /=2/g

foo(/= 2/g)

The first line divides the foo variable with 2/g. The second line calls the
foo function with the regex literal /= 2/g. Again, since jsTokens only
sees forwards, it cannot tell the two cases apart.

There are some cases where we can tell division and regex literals apart,
though.

First off, we have the simple cases where there’s only one slash in the line:

var foo =2/g

foo /=2

Regex literals cannot contain newlines, so the above cases are correctly
identified as division. Things are only problematic when there are more than
one non-comment slash in a single line.

Secondly, not every character is a valid regex flag.

var number = bar / 2/e

The above example is also correctly identified as division, because e is not a
valid regex flag. I initially wanted to future-proof by allowing [a-zA-Z]*
(any letter) as flags, but it is not worth it since it increases the amount of
ambigous cases. So only the standard g, m, i, y and u flags are
allowed. This means that the above example will be identified as division as
long as you don’t rename the e variable to some permutation of gmiyu 1 to 5
characters long.

Lastly, we can look forward for information.

If the token following what looks like a regex literal is not valid after a
regex literal, but is valid in a division expression, then the regex literal
is treated as division instead. For example, a flagless regex cannot be
followed by a string, number or name, but all of those three can be the
denominator of a division.

Generally, if what looks like a regex literal is followed by an operator, the
regex literal is treated as division instead. This is because regexes are
seldomly used with operators (such as +, *, && and ==), but division
could likely be part of such an expression.

Please consult the regex source and the test cases for precise information on
when regex or division is matched (should you need to know). In short, you
could sum it up as:

If the end of a statement looks like a regex literal (even if it isn’t), it
will be treated as one. Otherwise it should work as expected (if you write sane
code).